Titanium dioxide nano particle modified by surface stabilizer, titanium dioxide nano ink comprising the same, solar cell employing the same, and producing method of the same

ABSTRACT

Disclosed are a titanium dioxide nano ink having such a strong dispersibility as to be applicable by inkjet printing and having adequate viscosity without requiring printing several times, and a titanium dioxide nano particle modified by a surface stabilizer included therein. Inkjet printing of the titanium dioxide nano ink enables printing of a minute electrode. In addition, efficiency of a solar cell may be maximized since occurrence of pattern cracking is minimized.

CROSS-REFERENCE(S) TO RELATED APPLICATIONS

The present application is a Divisional Application of U.S. patentapplication Ser. No. 12/831,542, filed Jul. 7, 2010, which claimspriority of Korean Patent Application No. 10-2010-0024306, filed on Mar.18, 2010, which are incorporated herein by reference.

BACKGROUND

Technical Field

The present disclosure relates to a nano particle used to coat anelectrode of a dye-sensitized solar cell.

Description of Related Art

With the recent growing concerns on the global warming, development oftechnologies utilizing environment-friendly energy has been drawing muchof public attentions. Solar cell, being one of the most intriguingenergy sources as such, study on this field has been diversifiedincluding silicon-based solar cells, thin film solar cells usinginorganic materials such as copper indium gallium selenide (Cu(InGa)Se₂;CIGS), dye-sensitized solar cells, organic solar cells, andorganic-inorganic hybrid solar cells. Of them, the dye-sensitized solarcell, which is inexpensive and being drawn close to commercialapplication, has been highlighted in the fields of building-integratedphotovoltaics (BIPV) and portable electronics.

Unlike other solar cells, the dye-sensitized solar cell absorbs visiblelight and produces electricity through a photoelectric conversionmechanism. In general, patterning of the titanium dioxide workingelectrode used in the dye-sensitized solar cell is prepared by a screenprinting process. Screen printing is a printing technique in which ascreen is placed on a work table and a paste is applied on a substrateas it is being passed through a patterned mesh using a rubber bladecalled the squeegee. The screen printing process is, however,disadvantageous in that it requires a great amount of expensive pasteand it is applicable only to a flat substrate. Especially, the controlof pattern intervals is important since the efficiency of the solar cellincreases in proportion to the light receiving area. The limitation inthe control of linewidth between electrodes has been pointed out as theshortcoming of the screen printing technique.

Recently, there has been proposed to form electrode by inkjet printing.This method has advantages that it reduces material loss and has securedcontrol of narrow linewidths and its process is simple. The inkjet-basedpatterning process looks promising as a direct printing techniqueapplicable not only to flat-panel displays but also to solar cells andother applications.

The inkjet process is advantageous in that, since a wanted pattern canbe directly formed on a substrate using an inkjet head having smallnozzles, the number of processes and material consumption decrease ascompared to the screen printing technique and a desired pattern can becreated using a simple computer software. However, because a highlyviscous paste cannot be used in the inkjet method, printing has to beperformed several times to accomplish an electrode coating with apredetermined thickness.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY

The present invention relates to a titanium dioxide nano particlemodified by a surface stabilizer, a nano ink comprising the same, and adye-sensitized solar cell produced using the same.

An object of the present invention is to make ink-jetting in an inkjetprinting procedure easy by capping the surface of a titanium dioxidenano particle with a surface stabilizer.

Another object of the present invention is to provide a titanium dioxidenano ink comprising the titanium dioxide nano particle modified by asurface stabilizer, as well as additives such as an interfacialdispersant and a solvent, a substrate patterned using the titaniumdioxide nano ink, and a dye-sensitized solar cell produced using thetitanium dioxide nano ink.

The present invention provides a titanium dioxide nano particle coatedwith a surface stabilizer by chemical bonding so as to provide goodcompatibility with an ink composition, and a method for preparing thesame. The surface stabilizer may be represented by any one of ChemicalFormulae 1 to 3. The surface stabilizer has an acid functional group andalso has a hydrophobic moiety capable of providing stable dispersion inother materials.

In Chemical Formulae 1 to 3, R₁, R₂ and R₃ independently representhydrogen, C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₂-C₂₀ alkynyl or C₆-C₃₀ aryl.

The present invention also provides a nano ink comprising the titaniumdioxide nano particle capped with the surface stabilizer, a dispersantand a solvent.

The present invention further provides a substrate coated with the nanoink by inkjet printing, and a solar cell with an electrode layer printedusing the nano ink. The substrate or the electrode is free from theclogging problem because of minimized cohesion and minimized surfacetension on the nozzle, conferred by the surface stabilizer capped on thenano particle surface. Unlike the titanium dioxide thin film prepared byscreen printing process, pattern cracking during sintering may beminimized because the particles are uniformly distributed, which leadsto maximized diffusion and transition of electrons and improvedefficiency of a solar cell. Further, it is advantageous in that it isapplicable to a curved substrate since the inkjet process can be used.

The titanium dioxide nano particle of the present invention resolves theclogging problem since the capped surface stabilizer minimizes cohesionand minimized surface tension on the nozzle. The nano ink comprising thetitanium dioxide nano particle of the present invention may improveefficiency of a solar cell since occurrence of pattern cracking duringsintering is minimized and diffusion and transition of electronsproduced by a photoelectric conversion are maximized. Further, it isapplicable to a curved substrate since the inkjet process can beemployed.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the presentinvention will become apparent from the following description ofpreferred embodiments given in conjunction with the accompanyingdrawing, in which:

FIG. 1 shows a image of a nano ink comprising the titanium dioxideprepared in accordance with the present invention printed byink-jetting.

DETAILED DESCRIPTION

The advantages, features and aspects of the invention will becomeapparent from the following description of the embodiments withreference to the accompanying drawings, which is set forth hereinafter.

The present invention provides a titanium dioxide nano particle coatedwith a surface stabilizer by chemical bonding so as to provide goodcompatibility with an ink composition. The surface stabilizer may berepresented by any one of Chemical Formulae 1 to 3. The surfacestabilizer has an acid functional group and also has a hydrophobicmoiety capable of providing stable dispersion in other materials.

In Chemical Formulae 1 to 3, R₁, R₂ and R₃ independently representhydrogen, C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₂-C₂₀ alkynyl or C₆-C₃₀ aryl.

The titanium dioxide nano particle capped with the surface stabilizermay be obtained by reacting the surface stabilizer with titaniumisopropoxide, a precursor used to prepare a titanium dioxide nanoparticle. The solvent may be an alcohol, glycol, polyol, glycol ether,or the like. More specifically, methanol, ethanol, propanol,isopropanol, butanol, pentanol, haxanol, dimethyl sulfoxide (DMSO),dimethylformamide (DMF), glycerol, ethylene glycol, ethylene glycolmonomethyl ether, ethylene glycol monoethyl ether, ethylene glycoldimethyl ether, ethylene glycol diethyl ether, propylene glycol,propylene glycol propyl ether, etc., may be used alone or in combinationof two or more thereof. The proportion of the titanium isopropoxide, thesurface stabilizer and the solvent may be 5 to 8 vol %, 0.1 to 1 vol %and 91 to 94 vol %. Preferably, thus produced titanium dioxide colloidsolution has a titanium dioxide content from 10 to 15 vol %. Byevaporating the solvent from the titanium dioxide colloid solution, atitanium dioxide nano particle having a size of about from 3 to 30 nmmay be obtained.

The present invention further provides a nano ink comprising thetitanium dioxide nano particle capped with the surface stabilizer, adispersant and a solvent.

The dispersant is compatible with the surface structure of the nanoparticle and makes the nano particle disperse well in the solventwithout precipitating easily. The dispersant may be a non-ionicsurfactant. More specifically, it may be a polyethyleneoxide-polypropylene oxide block copolymer or a polyethyleneoxide-polystyrene block copolymer represented by Chemical Formula 4 or5.

In Chemical Formulae 4 and 5, n and m independently represent an integerfrom 1 to 30.

The copolymer represented by Chemical Formula 4 or 5 provides improvedlubrication at the interface with the titanium dioxide nano particle andthus is effective in improving dispersibility when it has a polyethyleneoxide (CH₂CH₂O) content from 30 to 80 wt % based on the total weight ofthe copolymer.

The solvent for the titanium dioxide nano ink may be an alcohol, glycol,polyol, glycol ether, etc. More specifically, it may be methanol,ethanol, propanol, isopropanol, butanol, pentanol, haxanol, dimethylsulfoxide (DMSO), dimethylformamide (DMF), glycerol, ethylene glycol,ethylene glycol monomethyl ether, ethylene glycol monoethyl ether,ethylene glycol dimethyl ether, ethylene glycol diethyl ether, propyleneglycol, propylene glycol propyl ether, or a mixture thereof.

In the titanium dioxide nano ink of the present invention, theproportion of the titanium dioxide nano particle, the dispersant and thesolvent may be about 10 to 70 parts by weight, about 0.1 to 10 parts byweight and about 20 to 82 parts by weight. If the content of thetitanium dioxide nano particle is less than 10 parts by weight, thenumber of inkjet printing has to be increased. Meanwhile, if it exceeds70 parts by weight, the ink may be inappropriate for inkjet printingbecause of too high viscosity. If the content of the dispersant is lessthan 0.1 part by weight, a desired effect may not be attained.Meanwhile, if it exceeds 10 parts by weight, the ink may beinappropriate for inkjet printing because of too high viscosity.

The titanium dioxide nano ink of the present invention may have aviscosity from about 1 to 50 cps at room temperature. If necessary, theink of the present invention may be heated to about 80° C. or belowduring application to reduce viscosity. By heating to 80° C. or below,the viscosity may be reduced to about 1 to 20 cps. The nano ink of thepresent invention may further comprise a viscosity modifier. Theviscosity modifier serves to modify the viscosity of the nano ink to beappropriate for printing.

The present invention further provides a solar cell with an electrodelayer printed using the titanium dioxide nano ink. After applying thetitanium dioxide nano ink on a substrate, the substrate may be sinteredto form an electrode pattern. The electrode pattern may be formed byinkjet printing. The inkjet printing method is advantageous in lessmaterial loss, easier control of narrow linewidths, a simpler process,or the like. Non-limiting examples of the substrate include a glasssubstrate, a transparent polymer substrate and a flexible substrate. Thesintering may be performed at about 300 to 500° C. for several minutesto several hours. During the sintering process, organic compoundsincluded in the titanium dioxide nano ink such as the dispersant and thesolvent are decomposed and destroyed, and the remaining titanium dioxidenano particles form a porous electrode.

EXAMPLES

The examples and experiments will now be described. The followingexamples are for illustrative purposes only and not intended to limitthe scope of the present invention.

Example Preparation of Titanium Dioxide Nano Particle and Manufacture ofSolar Cell Using the Same

Toluenesulfonic acid (1.72 mL) was dissolved in butanol (25 mL). Aftermixing butanol (150 mL) with Millipore water (5 mL) and adding titaniumisopropoxide (12 mL), the resultant mixture was added to thetoluenesulfonic acid solution. The mixture was reacted at roomtemperature for 1 hour and then at 110° C. for 6 hours. The reaction wasproceeded further by adding phenylsulfonic acid.

The solvent was evaporated from the resultant titanium dioxide colloidsolution to adjust the volume to about 120 mL.

Polyethylene oxide-polypropylene oxide copolymer (40:60, based onweight, 10 g) was added to the solution and then mixed. 1 hour later,the solution was treated with a tip-type sonicator for 10 minutes. FIG.1 shows an image of thus prepared titanium dioxide nano ink printed byink-jetting. It can be seen that the titanium dioxide nano particles aredispersed well with an interval of 200 μm.

The prepared nano ink was injected into a printer head and an electrodewas applied on a glass substrate. After heating at 300° C. for 1 hour,the substrate was sintered at 500° C. for 3 hours. After adsorbing a dye(N3, Solaronix) on thus prepared electrode for 24 hours at roomtemperature, it was bonded with a platinum counter electrode substrate(Surlyn, DuPont) at 120° C. After injecting an electrolyte through apreviously prepared hole, a dye-sensitized solar cell was completed byblocking the injection hole with Surlyn.

Comparative Example

A dye-sensitized solar cell was prepared according to a commonlyemployed method. A titanium dioxide paste (Solaronix) for screenprinting was coated on a fluorine-doped tin oxide (FTO)-coated glasssubstrate using a screen printing apparatus. After heating at 300° C.for 1 hour, the substrate was sintered at 500° C. for 3 hours. Afteradsorbing a dye (N3, Solaronix) on thus prepared electrode for 24 hoursat room temperature, it was bonded with a platinum counter electrodesubstrate (Surlyn, DuPont) at 120° C. After injecting an electrolytethrough a previously prepared hole, a dye-sensitized solar cell wascompleted by blocking the injection hole with Surlyn.

Current density (J_(sc)), voltage (V_(oc)), fill factor (FF) and energyconversion efficiency of the dye-sensitized solar cells according to theExample and Comparative Example were evaluated and compared, assummarized in Table 1. It can be seen that the present inventionprovides improved energy efficiency. Besides, the present invention isadvantageous in that it lowers production cost due to the decreased inkconsumption, has simplified process and applicability to a curvedsubstrate.

TABLE 1 Energy Current Fill factor conversion Samples density (J_(sc))Voltage (V_(oc)) (FF) efficiency (%) Example 4.09 0.623 0.679 1.73Comparative 3.95 0.622 0.655 1.61 Example

While the present invention has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

What is claimed is:
 1. A method for preparing a titanium dioxide nanoparticle ink, comprising: mixing and reacting titanium isopropoxide witha surface stabilizer represented by Chemical Formula 2 in a solvent toproduce a titanium colloid solution; evaporating the solvent from thetitanium colloid solution to produce titanium oxide nanoparticles cappedwith the surface stabilizer, and adding a dispersant which is acopolymer represented by Chemical Formula 5 in an amount from 0.1 to 10parts by weight relative to the total weight of the titanium dioxidenano particle ink to the titanium dioxide nanoparticles capped with thesurface stabilizer to obtain the titanium dioxide nano particle ink,

wherein R₂ represents hydrogen, C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₂-C₂₀alkynyl or C₆-C₃₀ aryl,

wherein n and m independently represent an integer from 1 to
 30. 2. Themethod of claim 1, wherein the solvent is an alcohol, a glycol or aglycol ether.
 3. The method of claim 2, wherein the solvent is methanol,ethanol, propanol, isopropanol, butanol, pentanol, haxanol, dimethylsulfoxide, dimethylformamide, glycerol, ethylene glycol, ethylene glycolmonomethyl ether, ethylene glycol monoethyl ether, ethylene glycoldimethyl ether, ethylene glycol diethyl ether, propylene glycol,propylene glycol propyl ether, or a mixture of thereof.
 4. The method ofclaim 1, wherein 5 to 8 vol % of titanium isopropoxide is mixed with 0.1to 1 vol % of the surface stabilizer and 91 to 94 vol % of the solvent.